Treatment or isolation of vascular aneurysms or of vessel walls which have been thickened by disease has traditionally been performed via surgical bypassing with vascular grafts. Shortcomings of this invasive procedure include the morbidity and mortality associated with major surgery, long patient recovery times, and the high incidence of repeat intervention needed due to limitations of the graft or of the procedure.
Minimally invasive alternatives involving stents or stent-grafts are generally known and widely used in certain types of treatments. Intralumenal stents, for example, are particularly useful for treatment of vascular or arterial occlusion or stenosis typically associated with vessels thickened by disease. Intralumenal stents function to mechanically hold these vessels open. In some instances, stents may be used subsequent to or as an adjunct to a balloon angioplasty procedure.
Stent-grafts, which include a graft layer either inside or outside of a stent structure, are particularly useful for the treatment of aneurysms. An aneurysm may be characterized as a sac formed by the dilatation of the wall or an artery, vein, or vessel. Typically the aneurysm is filled with fluid or clotted blood. The stent-graft provides a graft layer to reestablish a flow lumen through the aneurysm as well as a stent structure to support the graft and to resist occlusion or restenosis
Treatment of a bifurcation site afflicted with such defects as an occlusion, stenosis, or aneurysm is a particularly demanding application for either stents or stent-grafts. A bifurcation site is generally where a single lumen or artery (often called the trunk) splits into two lumen or arteries (often called branches), such as in a “Y” configuration. For example, one such bifurcation site is found within the human body at the location where the abdominal aortic artery branches into the left and right (or ipsalateral and contralateral) iliac arteries.
When a defect, such as an aneurysm, is located very close to the bifurcation of a trunk lumen into two branch lumens, treatment becomes especially difficult. One reason for this difficulty is because neither the trunk lumen nor either of the branch lumens provides a sufficient portion of healthy, lumen wall on both sides of the defect to which a straight section of single lumen stent or stent-graft can be secured. The stent or stent-graft must span the bifurcation site and yet allow undisturbed flow through each of the branch and trunk lumens.
What is required then is a stent or stent-graft which may be secured to each of the lumen wall a sufficient distance away from the defect and yet is capable of allowing undisturbed flow into each of the branch and trunk lumen. Such a configuration, at least after implantation, generally must have the same Y-shape as described for the bifurcation site. Prior to implantation, the stent or stent-graft may have a Y-shape or may have a modular construction which is assembled into the desired shape as it is implanted.
As we shall see, deployment of implants adapted to meet these needs is also problematic in that they must be deployed and secured in three different lumen which are impossible to access from a single direction. Further, to facilitate intralumenal delivery through a body's tortuous vasculature, the implant must be capable of being compressed into a very small diameter or profile.
Prior devices that deal with treatment at a bifurcation site within the body generally include grafts, stents, and stent-grafts in either a single-piece or modular configuration as described below.
The use of tubular grafts for treating defects at bifurcation sites has been known for some time. Bifurcated grafts have been disclosed, for example, in U.S. Pat. No. 3,029,819 to Starks, U.S. Pat. No. 3,096,560 to Liebig, U.S. Pat. No. 3,142,067 to Liebig, and U.S. Pat. No. 3,805,301 to Liebig. These grafts are typically made of woven fabric or other synthetic material and, because they have no supporting stent structure, typically involve excising the defected segment and suturing the fabric graft in place using common surgical procedures.
A number of bifurcated graft implants have been developed which use some limited means of supporting the one-piece bifurcated graft structure. Typically such means also provide a way to hold the graft open and secure the graft to the lumen wall without the need for sutures so that the graft may be suitable for translumenal delivery via a remote site as opposed to the more invasive surgical grafting techniques as described above.
One such implant is disclosed in U.S. Pat. No. 4,562,596 to Kornberg. Kornberg discloses a bifurcated aortic graft constructed for intraluminal delivery through the femoral artery. The graft consists of a tubular graft material having a plurality of longitudinal supporting struts or stays equipped with hooks or barbs to facilitate attachment within the desired location of the damaged artery. The graft is equipped with a resilient top ring for snugging the upper end of the trunk portion against the aortic wall upon deployment. The graft has a trunk portion and two legs, one of which is shorter than the other. The graft is delivered in a compressed state through a first branch artery with the shorter leg folded against the longer. At some point, as the compressed implant is advance through the first branch artery, the shorter leg clears the bifurcation point and articulates into the luminal region of a second branch artery. The entire implant is then moved in the reverse direction to allow the shorter leg to progress down the second branch. The implant is then expanded to its final shape against the lumen walls by way of a balloon.
Instead of longitudinal struts, other devices have provided the necessary support and securing means for the graft material by employing sections of stent structure at selected locations along the graft. Typically, stent rings may be placed at the trunk and/or branch openings of the stent. Such stent rings are either self-expanding or require balloon expansion.
U.S. Pat. No. 5,489,295 to Piplani et al., for example, discloses a bifurcated graft having a main body and legs formed of a flexible surgically implantable material. Expandable spring attachments are secured to the main body adjacent to the trunk opening and also to one of the legs adjacent to the branch leg opening. Delivered compressed, these expandable spring attachments urge the 25 openings of the graft open upon deployment and serve as anchoring means for securing the graft to the vessel wall. Lumen wall engagement may be enhanced by the addition of barbs or hooks at the apices of the spring attachments.
Another device involving a single-piece bifurcated graft with limited supporting and securing means is disclosed in U.S. Pat. Nos. 5,360,443 and 5,522,880 both to Barone et al. Barone et al. disclose a tubular aortic graft for intraluminal delivery. The graft is secured by the expansion and deformation of a thin-walled tubular member. The thin walled tubular member has a first diameter for delivery. Upon the application of an expanding force from a balloon, the member has a second expanded and deformed diameter for securing the graft to the lumen wall.
As with all such one-piece devices, the delivery of the graft implant is complicated by the fact that each of the trunk and two legs of the graft must be positioned into their respective lumen and then secured into place. This requires the branch legs to be compressed together for delivery through one of the lumen and requires difficult maneuvering of the branch legs to get them unfolded and untwisted into place within their respective branch lumen. This type of delivery requires the graft sections to be highly flexible so that its components may be manipulated as required and requires a larger profile. This demand for high flexibility often results in unsupported graft sections that may be subject to kinking, folding, collapse or the like.
Bifurcated stent devices generally suffer from even greater delivery problems because they are even less compressible and less flexible than their unsupported graft counterparts discussed above. U.S. Pat. No. 4,994,071 to MacGregor, for example, discloses a single-piece bifurcated stent for insertion into a bifurcating vessel. The stent includes a trunk portion constructed from a series of generally parallel loops interconnected by a sequence of half-hitch connections which extend along its length. The parallel loops form a cylindrical lattice which define a flow passageway. Two smaller cylindrical lattices are similarly constructed and attached to the trunk portion to form the branch flow passageways. The stent is designed to be delivered in one piece, over two guidewires (one for each branch) from the direction of the trunk vessel.
Another bifurcated stent example is found in U.S. Pat. No. 5,342,387 to Summers. Summers discloses a Y-shaped bifurcated stent comprising three coil sections (a major coil section and two minor coil sections), constructed of stent wire according to specific coil patterns and joined so as to form an unobstructed support for the trunk and branched vessels. To deploy the bifurcated stent, the stent is compressed around an inflarable bifurcated balloon. The compressed balloon/stent is delivered up one of the branch vessels until one of the minor coils is clear of the bifurcation juncture. Then the stent is backed down, positioning one of the minor coils in each of the branches. The stent is then expanded by the bifurcated “tri-wing” balloon and then the balloon is removed.
To alleviate these complicated delivery problems, some implant devices have used a modular approach. An example of a modular stent may be found in FR 2,678,508 A1. According to that disclosure, in order to provide continuity at junctions, in particular at vessel bifurcations, at least two helicoidal spring elements comprised of self locking coils are used. A first element is constructed to have a first coil diameter section corresponding to the diameter of the aortic artery and a second coil diameter section corresponding to one of the iliac arteries. A second element similarly has two sections, one of which corresponds to the diameter of the second iliac artery, the other equivalent to the diameter of the aortic section of the first element so that the corresponding coils of each element may be delivered separately and locked together in situ.
Another type of modular approach involves using two separate elongated tubes delivered through each of the branch lumen. The tubes, which may consist of a tubular graft element supported by unconnected stent segments, establish the required flow lumen in each of the branch arteries, and in the trunk lumen, are forced together in such a manner as to substantially seal around the periphery. Generally, the shape of the two tubes in the trunk lumen is that of two back-to-back, semi-circular “D” shapes. U.S. Pat. No. 5,507,769 to Marin et al and EP 0 551 179 A1 to Palmaz et al. disclose implants of the type which employ two separate tubes. Palmaz further discloses the use of an expandable tubular member which is secured to the trunk artery prior to delivery of the separate tubes. The purpose of this additional member is help secure the two tubes to the trunk lumen.
Still another type of modular stent-graft device is disclosed in WO 95/21592 (International Application number PCT/US95/01466). In that publication, there is disclosed a modular bifurcated stent or stent-graft comprising two separate modules for delivery in a compressed state and connected together in situ. A first module has a proximal part adapted to engage the trunk artery, then bifurcating into a first distal portion adapted to extend into one branched artery and a female receiving opening to be positioned near the other branched artery. A male proximal portion of a second module is connected to the female receiving opening. According to the disclosure, to ensure the two modules remain connected, it is preferred that the receiving opening has a frustoconical section and the second module has a mating male frustoconical section.
Further related modular stent-graft approaches may be found for example in EP 068 6 379 A2, EP 0 696 447 A2, and U.S. Pat. No. 5,562,724 to Vorwerk et al. While these modular devices tend to offer a measure of improved delivery, continuing problems may include a certain amount of leakage around the openings of the device, leakage at the modular connection, increased compressed profiles, and unoptimal flexibility, kink-resistance, and axial stiffness.
Further it is generally important for any stent or stent-graft to be accurately and quickly deployed so that it may be properly positioned at a desired treatment location. This is especially true with bifurcated devices using a modular approach because, when deployed, the connecting feature must be properly aligned rotationally over a branch artery.
From the foregoing discussion it is evident that it would be desirable to have a self-expanding stent-graft device possessing superior kink-resistance and flexibility to allow the device to follow the natural geometry of the vasculature and, at the same time, allow for sufficient axial stiffness to facilitate accurate placement, resist movement, and prevent leakage. It would be highly desirable to have such a stent or stent-graft device that also provides for a reduced compressed profile for delivery through the body's vasculature. It would be highly desirable for such a stent or stent-graft to have the capability to ensure axial and rotational alignment.